The present invention relates to fans, and more particularly to flexible fan blades that operate over a large range of speed and pressure.
In conventional fan assemblies, a highly pitched, fixed-wing fan blade is efficient at low differential pressure with high output flow. However, the same highly pitched, fixed-wing fan blade stalls as the output flow approaches zero. At the point of stall, as the output flow decreases, the power input increases while the pressure increases very little or may decrease. This is equivalent to the stall of an airplane wing. When the angle of attack increases beyond a critical point, airflow across the top of the wing separates from the wing and continues without being deflected downward with the wing. Thus, because the airflow on the upper surface of the wing is not pulled downward by the wind, the wing is not pulled upward by the airflow above the wing. Thus, the plane loses lift, though the airflow on the lower surface of the wing continues to provide some lift as it is deflected downward.
For other fan assemblies, a low pitched, fixed-wing fan blade is efficient at high differential pressure with low output flow. No stall occurs. However, at low differential pressure, the same fan is inefficient and the output flow is low. The fan speed may be increased to increase the output flow, but the additional fan blade drag keeps the efficiency low and the power input high.
One design is to allow for variable pitch in the fan blade and hub assembly. This design provides for rotation of the fan blade along its longitude, thereby controlling the pitch. However, additional mechanisms must be provided to control the pitch according to differential pressure and/or fan speed. One disadvantage of this design is that the solid blade has a fixed helical twist (high pitch angle near the fan hub and lower pitch angle near the blade wingtip). The predetermined, helical twist is optimized for a particular angular position of the blade. As the solid blade is rotated to reduce the pitch under high differential pressure conditions, the pitch angle is reduced by the same amount along the length of the blade. Therefore, the pitch at the wingtip is overcompensated relative to the blade's pitch near the fan hub. Another disadvantage is the cost and maintenance of the mechanism to rotate each of the fan's blades, as well as the systems to control the rotation. Also, failure of these mechanisms and systems can cause great loss in critical, high-value applications.
Another design is to allow for flexibility in the wing of the fan blade itself. Some fans combine a rigid leading edge element with a curved, flexible wing element. The curved (cambered), flexible wing element trails the rigid leading edge and is sandwiched between and upper and lower portion of the rigid leading edge. The rigid leading edge is set at a fixed pitch. As the fan speed increases, thereby increasing the differential pressure (given the fixed system resistance coefficient), the flexible wing element is deflected away from the higher pressure side (the “lower” side as viewed as an airplane wing). The greatest degree of bending in the flexible wing element occurs where this flexible wing element connects to the rigid leading edge. Preloading (biasing) elements and/or limiters are provided to reduce localized stress and vibration, both of which could lead to failure.
One disadvantage of the above design is that the overall camber of the wing is more significantly reduced by the high differential pressure than the overall pitch of the wing. Thus, the lift that creates the differential pressure, generated by the angle of attach of the wing, is much greater than the lift generated by the camber of the wing under high differential pressure. Thus, this flexible fan blade can stall occur under high differential pressure, low flow conditions. Another disadvantage of this design is that the flexible wing element rubs against the preloading elements and/or limiters as it bends under high and low differential pressure or vibrates. Additionally, the preloading elements and/or limiters, located on the upper wing surface, affect the airflow over the airfoil and can contribute to the separation (stall) of airflow over the upper wing surface.
Yet another conventional design is a flexible fan blade that attaches directly to the fan hub, thus fixing both the camber and pitch of the wing near the fan hub. Between the fan hub and the wingtip, the leading edge is relatively rigid, while the curved, flexible trailing wing portion is deflected by the differential pressure. The fan wing is typically of one piece construction. While this design solves the problem of localized stress, rubbing and perturbed airflow as in the other designs described above, the wing pitch near the fan hub is fixed and can stall in this area. Also, the wingtip is subject to deflecting and vibrating about the blade's longitude, therefore limiting the safe speed and pressure differential of the fan.
Still yet another design includes a fan blade of flexible material attached to a rigid leading edge and includes materials of differing thermal expansion coefficients, whereby the blade curvature is increased by higher temperature and decreased by lower temperatures and aerodynamic lift on the blade. This type of fan is directed toward cooling of internal combustion engines. However, as with the other prior art designs, the overall camber of the wing is more significantly reduced by the high differential pressure than the overall pitch of the wing.